1. Field of the Invention
This invention broadly relates to a mill blank assembly used in the field of dentistry to create an inlay, onlay, crown, veneer, coping, bridge, bridge framework, implant, implant abutment or other restoration or restoration component. More specifically, the present invention is directed to a mill blank assembly that is especially adapted for use with computer-aided design and machining processes to create a dental prosthesis. The present invention is also directed to a method for making a dental mill blank assembly.
2. Description of the Related Art
A variety of dental procedures are known for replacing or repairing damaged, weakened or missing tooth structures. For example, a dental prosthesis commonly known as a filling is often used to fill cavities in teeth caused by tooth decay or caries. Somewhat larger prosthetics also used to fill cavities are known as inlays and onlays. Fillings, inlays and onlays may also be utilized to restore the shape of teeth that have been chipped or broken.
Other types of dental prosthetics include bridges, full crowns and partial crowns. Typically, these prosthetics are much larger than fillings and as a result are often more visible in the oral cavity. Full and partial crowns may be supported by remaining portions of the original tooth structure and/or by a post extending toward the bony region of the jaw. Bridges, on the other hand, are structures that connect to adjacent tooth structure and provide an artificial tooth or tooth crown to replace corresponding, missing structure.
In the past, fillings and some inlays and onlays were often made of a silver-colored metal alloy known as amalgam due to its relatively long life and relatively low cost. Another advantage offered by amalgam is that it allows a dental practitioner to fit and fabricate the restoration during a single session with a patient. Unfortunately, amalgam is not considered aesthetic since its silver color sharply contrasts to the appearance of natural teeth in the oral cavity.
Another material used for dental prosthetics, and particularly for larger inlays and fillings, is gold. However, like amalgam, the color of gold sharply contrasts with the appearance of natural teeth and is highly visible in the oral cavity. In addition, gold is relatively expensive in comparison to other dental materials.
As a consequence, many dental practitioners are increasingly turning to ceramic or polymer-ceramic composite materials for use to make dental prosthetics. Dental ceramic materials and dental polymer-ceramic composite materials can provide an appearance that closely matches the appearance of natural teeth. Such materials are also available in various color shades so that the practitioner can select a color that closely matches the color of adjacent tooth structure.
Dental polymer-ceramic composite materials for use as restoratives are available from various manufacturers in paste-type form. Such materials are often supplied in capsules that are releasably received in a receptacle of a hand-held dispenser. The dispenser typically includes a lever that, when depressed, extrudes a quantity of the material from the capsule and directly onto the tooth structure. The material includes a polymerization initiator that serves to harden the material once it has been placed on the tooth structure and shaped by the practitioner to resemble natural tooth structure.
A variety of techniques may be employed to help shape the unhardened restorative paste to a desired configuration once dispensed onto the patient""s tooth structure. For example, if the material is used to fill a relatively small cavity, the material can be dispensed directly into the cavity and then shaped by hand. A hand instrument such as a dental pick is used to help pack the material in the cavity and to blend the external surface of the paste with adjacent, external portions of the patient""s tooth. As another example, if a portion of one or more sides of a tooth is to be restored, the practitioner may elect to use a matrix band or sectional matrix band next to the tooth structure to help hold the material in place while it hardens. The matrix band or sectional matrix band serves as a formwork, similar to formwork used in concrete, to help hold the material in place and also to help define an outer surface of the composite material while it hardens.
However, larger prosthetics are often fabricated outside of the oral cavity and then placed in the patient""s oral cavity once completed. For these types of prosthetics, an impression is often taken of the patient""s tooth structure of interest along with adjacent regions of the gingiva, using an elastomeric impression material that provides a negative physical image of the tooth structure and gingival region. Next, a cast positive model is made by pouring a quantity of plaster of Paris into the impression and allowing the plaster of Paris to harden. The resulting plaster of Paris or xe2x80x9cstonexe2x80x9d model is then used in the laboratory to make a prosthetic that is ultimately transferred to the patient""s oral cavity.
The laboratory procedure for making the prosthetic may be somewhat involved, depending on the type of prosthetic that is needed. In one method, for example, a wax replica of the desired crown is built on the stone model. The wax replica is then embedded in a refractory investment material and fired to create another negative physical image of the oral structure of interest. Porcelain is then forced into the investment material under pressure and heat in order to make the crown.
However, a number of disadvantages arise when the foregoing procedure is followed to make a crown. In such a procedure, the patient typically travels to the practitioner""s office two times: a first time to enable an impression to be taken, and a second time a few days later after the stone model has been made and the crown has been fabricated in the dental laboratory. Moreover, if the completed crown must be returned to the laboratory because its shape, fit or appearance is not satisfactory, the patient is often then required to return to the dental office for a third visit. In many dental practices, the crown is not made in a laboratory that is part of the office but is instead sent to a central laboratory in another area of the town or region.
Furthermore, the fabrication of custom dental crowns and other prosthetics by hand from stone models is an art that involves a high degree of skill and craftsmanship, as well as intensive labor. Moreover, prosthetics that are placed in the anterior regions of the patient""s oral cavity are often highly visible. It is widely considered difficult to make a porcelain prosthetic that exactly matches the translucency and color of natural teeth.
Recently, increased interest has been directed toward the use of computer automated machinery for fabricating dental prosthetics, using far less labor than prior methods such as the method for making a crown described above. For example, several systems are known for collecting a set of electronic data that is representative of the patient""s tooth structure of interest. The data is then used by an automated mechanical milling machine (such as computer-aided milling machine) to fabricate a prosthetic that, when completed, closely matches the shape of natural tooth structure.
Examples of computer-aided milling machines used in the field of dentistry include the CEREC 2(trademark) and CEREC 3(trademark) machines available from Sirona Dental Systems of Bensheim, Germany, the DICEM(trademark) machine from Dentronix, the VITA CELAY(trademark) machine from Vita Zahn Fabrik of Bad Sackingen, Germany, the PRO-CAM(trademark) machine from CadCam Ventures, of Dallas, Tex. and the PROCERA ALL CERAM(trademark) machine from Nobel Biocare USA of Westmont, Ill. U.S. Pat. Nos. 4,837,732, 4,776,704 and 4,575,805, as well as PCT Patent Application No. WO 96/37163 also disclose systems for making dental prosthetics using computer-aided milling machines.
The fabrication of a dental prosthesis using a computer-aided machining system typically involves the use of a xe2x80x9cmill blankxe2x80x9d, a block of material from which the prosthetic is cut. Dental mill blocks are often made of a ceramic material. Commercially available dental mill blanks include VITA CELAY(trademark) porcelain blanks from Vita Zahn Fabrik, VITA NCERAM(trademark) ceramic blanks from Vita Zahn Fabrik, MACOR(trademark) micaceous ceramic blanks from Coming, and DICOR(trademark) micaceous ceramic blanks from Dentsply. A dental mill blank made of a ceramic silica material as described in U.S. Pat. No. 4,615,678. An improved ceramic dental mill blank is described in applicant""s co-pending PCT application entitled xe2x80x9cCERAMIC DENTAL MILL BLANKSxe2x80x9d, PCT US00/19887, filed Aug. 26, 1999.
Dental mill blanks may also be made of resinous materials. An example of a dental mill blank made of a polymeric resin and a filler is described in applicant""s co-pending PCT patent application entitled xe2x80x9cDENTAL MILL BLANKSxe2x80x9d, PCT US99/10966, filed Jan. 8, 1999. Dental mill blanks made of such material exhibit superior milling characteristics such as hardness and cutting properties relative to previously known dental mill blanks.
Many commercially available dental mill blanks are made of a two-piece construction that comprises a support stub section and a milling blank section. The support section is cylindrical and adapted to fit into a collet or a Jacobs chuck of a milling machine. Often, the support section is made of metal, since the support section is ultimately detached from the milling section and does not form part of the finished prosthetic. The support section is typically made of a relatively soft metallic material such as an aluminum alloy that is easy to machine to precise tolerances.
The milling section of conventional two-piece dental mill blank assemblies is often made of one of the aesthetically-pleasing restorative materials described above so that the resulting prosthetic provides a natural appearance once placed in the oral cavity. The milling section of conventional assemblies has a flat face that is joined to a flat face of the support section by an adhesive. An example of one type of two-piece construction is described in U.S. Pat. No. 4,615,678.
It has been observed, however, that conventional dental mill blank assemblies occasionally fracture during the milling process. In some instances, the fracture occurs in the joint between the support stub section and the milling section. It is suspected that lateral forces exerted by the milling tool on the milling section create a shear force that exceeds the strength of the adhesive bond of the joint.
Unfortunately, if the milling section has broken away from the support section before the milling process has been completed, the mill blank assembly must be discarded and replaced with a new assembly. Consequently, the fracture of dental mill blank assemblies represents a time-wasting nuisance to the personnel operating the milling system and possibly to the patient. Replacement of the dental mill blank assembly with a new assembly also represents an additional cost to the dental laboratory, the dental practitioner and the patient that is best avoided if at all possible.
Applicant""s previously filed patent application, U.S. Ser. No. 09/653,230 entitled xe2x80x9cDENTAL MILL BLANK AND SUPPORT STUB ASSEMBLYxe2x80x9d, now U.S. Pat. No. 6,482,284B1, describes an improved two-piece mill blank assembly. In that assembly, a projection extends from the milling section or the support section into the remaining section and helps resist unintentional detachment of the sections from each other during a machining operation. While the inventions described in that patent application represent a significant improvement over past practice, there is a continuing need to further advance the state of the art with respect to dental mill blank assemblies.
In accordance with the invention, a dental mill blank assembly includes a milling section and a support section, and the support section is also part of a mold assembly that is used to form and make the milling section during manufacture of the assembly. In certain embodiments of the invention, the milling section is fixed to the support section by a chemical bond that begins to form within the mold cavity, such that the milling section and the support section are adhered to each other at the time that the mold assembly is opened for removal of the mill blank assembly. The use of an additional adhesive component to bond the milling section to the support section is unnecessary.
The dental mill blank assembly of the present invention is especially suitable for use in computer-aided machining systems such as the machining systems described above. The mill blank assembly presents an enhanced resistance to fracture during the time that the milling section is machined in a milling system, and safely resists forces exerted by a milling tool in lateral directions so that the dental prosthetic can be fully machined. The chemical bond between the support section and the milling section greatly reduces the likelihood that the milling section is detached from the support section during a machining operation.
In more detail, the present invention in one aspect is directed to a mill blank assembly for a dental prosthesis that comprises a milling section made of a material suitable for making a dental prosthesis. The mill blank assembly also comprises a support section having a shaft for releasably supporting the mill blank assembly in a milling machine. The support section is bonded to the milling section by a chemical bond directly between the material of the support section and the material of the milling section.
Another aspect of the invention is directed toward a dental mill blank assembly that consists essentially of a milling section and a support section. The milling section is made of a material having a composition that is different than the composition of the support section.
The present invention is also directed toward a dental mill blank assembly that includes a support section having a shaft for releasably supporting the mill blank assembly in the milling machine. The support section includes a through passageway. The mill blank assembly also includes a hardened restorative material connected to the support section. A portion of the hardened restorative material is located within the passageway. Another portion of the hardened restorative material is located externally of the passageway, serves as a milling section and is integrally connected to the portion of hardened restorative material that is located within the passageway.
Another aspect of the present invention is directed toward a method of making a dental mill blank assembly. This method includes the act of providing a mold assembly that includes a first mold component, a second mold component and a mold cavity defined at least in part by the first mold component and the second mold component. The method also includes the act of directing a quantity of restorative material into the mold cavity. The method further includes the act of reacting the restorative material with the second mold component in order to establish a chemical bond between the restorative material and the second mold component. The method also includes the act of hardening the restorative material located in the mold cavity.
Another aspect of the present invention is directed toward a method of making a dental prosthesis. The method includes the act of providing a mold assembly that includes a first mold component, a second mold component and a mold cavity defined at least in part by the first mold component and the second mold component. The method also includes the acts of directing a quantity of restorative material into the mold cavity, and establishing a bond between the restorative material and the second mold component. The method further includes the acts of hardening the restorative material located in the mold cavity, and separating the first mold component from the restorative material. The method additionally includes the acts of mounting the second mold component in a milling machine, and milling the hardened restorative material at least partially into a prosthesis.
The present invention is also directed in an additional aspect to a method of making a dental mill blank assembly. In this aspect, the method includes the act of directing a quantity of restorative material through a passageway of a support section. The method also includes the act of hardening restorative material located within the passageway as well as restorative material located externally of the passageway, such that the hardened restorative material located externally of the passageway serves as a milling section and the hardened restorative material located within the passageway resists detachment of the milling section from the support section.
Other aspects, features and advantages of the invention are described in the paragraphs that follow and are illustrated in the accompanying drawings.